<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>$课堂练习$</title>
</head>
<body>
<a href="http://www.qq.com" id="ak">QQ</a>
<input type="button" value="点我" id="btn">
<input type="text" value="文本框" id="txt">
<input type="button" value="变色" id="btn1">

<ul id="xx">
    <li>哈哈</li>
    <li>哈哈</li>
    <li>哈哈</li>
</ul>
<script>
    document.getElementById("btn").onclick = function () {
        document.getElementById("txt").disabled = true;

    };
    document.getElementById("ak").onclick = function () {
        alert("点我干嘛");
        return false;
    };
    document.getElementById("btn1").onclick = function () {
        document.getElementById("xx").style.backgroundColor = "yellow";
    };
</script>
</body>
</html>